Magnesium base alloys



United States Patent 3,416,978 MAGNESIUM BASE ALLOYS Edward FrederickEmley and Peter Clive Meredith, Clifton Junction, near Manchester,England, assignors to Magnesium Elektron Limited, Clifton Junction, nearManchester, England, a British company No Drawing. Continuation ofapplication Ser. No. 613,967, Feb. 3, 1967, which is a continuation ofapplication Ser. No. 352,322, Mar. 16, 1964. This application Mar. 19,1968, Ser. No. 714,359 Claims priority, application Great Britain, Apr.3, 1963, 13,197/ 63 1 Claim. (Cl. 148-32.5)

This application is a continuation of US. application Ser. No. 613,967filed Feb. 3, 1967, now abandoned, which is in turn a continuation ofUS. application Ser. No. 352,322 filed Mar. 16, 1964 and now abandoned.

This invention relates to magnesium base alloys'and its main object isto produce an alloy which in the fully heat treated cast state has acombination of mechanical strength properties hitherto unobtained and inparticular a 0.1% proof stress of over 12.5 tons per square inch on testbars made according to British standard specification No. L101 combinedwith adequate maximum tensile stress (e.g. at least 17 tons per squareinch) and adequate elongation (e.g. at least 3 percent elongation).

In the specification of British Patent 511,137 it was proposed toincorporate zirconium in order to achieve fine grain size and highstrength and commercial alloys were developed on this basis including inparticular an alloy containing 0.4 to 1.0 percent zirconium and 4 to 6percent zinc. Fully heat treated cast test bars of such alloys havegiven about 911 tons 0.1% proof stress, 16-17 tons tensile and 28%elongation. The specification mentions a list of metals (includingsilver) which would not precipitate the zirconium from the molten alloybut no suggestion was made that these other than zinc and cadmium wouldoffer any improvement in mechanical properties.

In the further development of these alloys we developed the alloy (knownas TZ6) of British Patent 734,011 containing zirconium with 4 to 8percent zinc and at least 0.5 percent thorium. This alloy in the as caststate gives a 0.1% proof stress of 8.5 tons with 16-17 tons tensilestress. The properties of this alloy can be improved by precipitationheat treatment whereupon the 0.1% proof stress is increased to 9.5 to11.0 and the tensile stress is increased to 17.0 to 19.5 with elongationto 15 percent. However, this alloy cannot be improved by solution heattreatment.

Another high strength magnesium alloy (known as MSR) described in thespecification of British Patent No. 875,929 consists of zirconium up to1.0%, 0.5 to 3.5 rare earth metals and from 1.5 to 3.5 percent silver,with less than 0.5 percent zinc if any. More than 0.5% zinc in this MSRalloy spoils the properties of the alloy. This alloy can achieve a 0.1%proof stress of 12.0 tons with an ultimate tensile stress of 16.0 tonsand 3% elongation.

According to the present invention we provide an alloy consisting apartfrom impurities of:

Percent Zinc (preferably 3-7) 3 to 8 Silver (preferably 1.54.0%) 1 to4.0 Zirconium 0.4 to 1.0 Thorium 0 to less than 0.5 Rare earth metals 0to less than 0.5 Thorium and rare earth metals together less than 0.5Manganese 0 to 0.15 Cadmium 0 to 1.0 Calcium 0 to 0.8 Indium 0 to 2.0

Thallium 0 to 5.0

3,416,978 Patented Dec. 17, 1968 Percent Lead 0 to 1.0 Beryllium 0 to0.01 Magnesium Balance For highest strength with full zirconium contentwe may use a zinc content of 5 to 8 percent whereas for alloys havinggreater ductility we may use 3.5 to 5.0 percent zinc.

For alloys containing 5 to 7% zinc, heat treatment may be carried out byfirst effecting solution heat treatment at 450 to 490 C. for at leasttwo hours, followed by precipitation heat treatment at a temperature ofto 180 C. for a period of at least 5 hours (preferably 10 to 30 hours).For alloys containing 3 to 5% zinc the solution heat treatmenttemperature can be as high as 520 C.

It is surprising that whereas an alloy consisting of 4 to 5 percentzinc, zirconium, balance magnesium, cannot be materially improved byfull heat treatment, the addition of 1.5 to 4.0 percent silver resultsin the alloy being somewhat improved by full heat treatment.

After effecting solution heat treatment the alloy should be rapidlycooled. It is found that the aforesaid MSR alloy if cooled with an airblast at this stage suffers loss of mechanical properties and musttherefore be quenched in liquid which is less convenient. However, thealloy of the present invention can be cooled by an air blast at thisstage and still develops good mechanical properties.

As examples of the mechanical properties achieved by the presentinvention, the following test results may be cited:

Alloy A1 Percent Zinc 6 Ag 3 Zr 0.75

Sand cast test bars to B.S.L.101 type B.

(a) Solution heat treated for 16 hours at 460 C., air blast cooled andaged at 150 C. for 24 hours.

(b) Solution heat treated .for 16 hours at 460 C., air blast cooled andaged at 125. C. for 96 hours.

Sand cast test bars to B.S.L.101.

(a) Solution heat treated for 7 hours at 480 C., air blast cooled andaged for 96 hours at C.

(b) Solution heat treated for 2 hours at 490 C., air blast cooled andaged for 16 hours at C.

0.1% proof stress, Ultimate tensile Elongation,

tons/sq. inch stress, tons/sq. inch. percent on 2 inches.

Alloy A3 Percent Zn 7 Ag 2 Zr 0.75

ASTM type test bars solution heat treated 16 hrs. at 460 C., air blastcooled, and aged for 72 hrs. at 125 C.

0.1% proof stress, tons/sq. inch 14.9 0.2% proof stress, tons/sq. inch16.3 Ultimate tensile stress, tons/sq. inch 21.4

Elongation, percent on 2 in /2 We have further found that theMg-Zn-Ag-Zr alloys difier from the other zirconium bearing castingalloys in that whereas, as is well known, full saturation of the metalwith zirconium is desirable to suppress micro-porosity, the preferredalloys of the present invention give maximum soundness with a dissolvedzirconium content significantly below the saturation level. Such alloyswith reduced dissolved zirconium content show greater response to theuse of chills and show higher tensile properties in castings owing tothe greater soundness, despite slightly coarser grain size arising fromthe lower dissolved zirconium content.

The increased soundness obtainable with the reduced zirconium contentenables higher zinc contents to be effectively utilised than arepreferable when the alloys are saturated with zirconium. The usefulrange of zinc content is then extended, e.g. to about 8%. To providegood ductility at the higher zinc levels, the preferred silver contentmay then be reduced, e.g. to about 1%. Solution heat treatmenttemperatures will generally be slightly lower for alloys with reducedzirconium content.

To produce Mg-Zn-Ag-Zr alloys with a controlled lower zirconium content,one convenient procedure is to melt down metal charges consisting ofpre-alloyed ingot (containing e.g. 0.l-0.3% Zr) and Mg-Zn-Ag-Zr alloyscrap, and to revivify each melt with a 3% addition of Mg -30% Zrhardener alloy. For full saturation with zirconium, a 5% addition ofsuch hardener would normally be used.

The following data illustrate the increased tensile prop- SolubleDistance Tensile properties (full size test bars) zirconium fromanalysis, chill 0.1% 0.2% Ultimate Elong.

percent (in.) proof proof stress percent; stress stress on 2 in.

The alloys of the present invention may be used for cast parts requiringhigh strength such as aircraft frame parts but may also be used inwrought forms.

The magnesium content of the alloy should be at least percent preferablyat least percent.

The alloy of the kind containing this reduced quantity of solublezirconium will usually contain up to 8 percent zinc (e.g. 5 to 8percent) together with 0.4 to 0.7 (preferably 0.5 to 0.7 percentzirconium. The soluble zirconium is soluble in dilute hydrochloric acidas distinct from any additional small amount of zirconium which may bepresent but which is not soluble in dilute hydrochloric acid. The normalsoluble zirconium content of alloys at the 58% zinc level is over 0.7,e.g. 0.75 to 0.9 percent.

We claim:

1. A solution heat treated and precipitation heat treated cast articleconsisting essentially of 5 to 7% Zinc, 1.5 to 4% silver, 0.4 to 1%zirconium, balance magnesium.

References Cited UNITED STATES PATENTS 2,178,580 11/1939 Gann 751682,750,288 6/1956 Jessup et a1 75-168 3,039,868 6/1962 Payne et al75--161 FOREIGN PATENTS 511,137 8/1938 Great Britain.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R.

1. A SOLUTION HEAT TREATED AND PRECIPITATION HEAT TREATED CAST ARTICLE CONSISTING ESSENTIALLY OF 5 TO 7% ZINC, 1.5 TO 4% SILVER, 0.4 TO 1% ZIRCONIUM, BALANCE MAGNESIUM. 